HIGH-PRESSURE SINGLE-CRYSTAL X-RAY-DIFFRACTION AND INFRARED SPECTROSCOPIC STUDIES OF THE C2 M-P2(1)/M PHASE-TRANSITION IN CUMMINGTONITE/

The structural changes associated with the C2/m-P2(1)/m phase transiti on in cumming-tonite with (Fe + Mn)/(Fe + Mn + Mg) approximate to 0.50 have been studied with single-crystal Xray diffraction at various pre ssures up to 7.90 GPa and infrared spectroscopy up to 8.63 Cpa. With i ncreasing pressure, the crystal transforms from C2/m to P2(1)/m symmet ry at similar to 1.21 GPa, as determined by the appearance of reflecti ons violating the C2/m space group. Infrared spectra provide additiona l evidence for the phase transition: A distinct splitting of OH stretc hing bands results from an increase from one to two nonequivalent OH p ositions. The C2/m-P2(1)/m transition is of weakly displacive first-or der or tricritical character with apparent slope changes in the plots of the axial ratios alb and nle as a function of pressure. The unit-ce ll compression is considerably anisotropic with the a dimension in bot h C2/m and P2(1)/m phases being the most compressible. Major structura l changes for the C2/m-P2(1)/m transition include: (1) One crystallogr aphically distinct silicate chain becomes two discontinuously, coupled by the splitting of the M4-O5 bond, as well as M4-O6, into two nonequ ivalent bonds, and (2) the M4-cation coordination increases from sixfo ld to sevenfold. More importantly, we observed a change in the sense o f rotation for the A chain while the crystal structure maintains P2(1) /m symmetry: It is O rotated, as the B chain, at 1.32 Gpa, but S-rotat ed at 2.97 GPa and higher pressures. As pressure increases from 1.32 t o 7.90 Gpa, there is a switching of the nearest bridging O atoms coord inated with the M4 cation: The M4-O5B distance contracts from 2.944 to 2.551 Angstrom, whereas the M4-O6B distance increases from 2.754 to 2 .903 Angstrom. Compression mechanisms for the low- and high-pressure p olymorphs appear to be slightly different. In the C2/m phase, the beha vior of the A and M4 sites controls the compression of the structure, whereas the response of the M1, M2, and M3 octahedra to pressure also plays a role in determining the compression of the P2(1)/m structure. The phase transition is regarded as primarily driven by the differenti al compression between the M4 and T sites, and the symmetry breaking p rovides a necessary tighter coordination for the M4 site, Based on our data, the obvious changes in the hyperfine parameters of Fe-57 in gru nerite between 1.0 and 3.4 GPa, observed by Zhang and Hafner (1992), a re likely to result from the C2/m-P2(1)/m structural transformation.